After mining out all the ore in several mines, I returned (after a month which should be plenty of time for the ore to re-spawn) only to find that I still can not mine the ore. I can tell that it has re-spawned because I can see that the coloration has returned, but I am not able to interact with the ore in any way. Why can't I mine the ore any more?
You have a collision problem. I think the respawn is actually a week, but if you're on PC, just open console, select the ore vein, hit disable, then enable, and you should be able to mine. It's a glitch. Nothing you can do if you're on console, you'll just have to wait for a patch.
The colouration returned + can't use = Glitch. Try walking into it, you'll clip right through.